Metal-air battery cells include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. A metal-air battery has a relatively high energy density because the cathode utilizes oxygen from the ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. Metal-air battery cells are often arranged in a common housing for a multiple-cell battery pack. Such a multiple-cell battery pack provides a relatively lightweight battery with a significantly long run time.
A steady supply of oxygen is necessary for the metal-air battery cells to provide electricity. Known metal-air batteries typically use an air manager system of a fan within a housing containing the cells. The fan sweeps a continuous flow of ambient air across the cells at a flow rate sufficient to achieve the desired power output. Such an arrangement is shown in U.S. Pat. No. 4,913,983 to Cheiky. Cheiky uses a fan within a battery housing to supply a sufficient flow of ambient air to a pack of metal-air cells. When the battery is turned on, an air inlet and an air outlet are opened and the fan is activated to create the flow of air into, through, and out of the housing.
Although a sufficient flow of ambient air is needed for the metal-air cells to produce electricity, the level of humidity in this air flow can have a significant impact on the operation of the cells. Once exposed to the ambient air, the humidity level within the battery housing will seek equilibrium with the ambient humidity level. The relative humidity within a metal-air battery housing is generally about forty-five percent (45%). If the ambient humidity level is greater than the humidity level within the battery housing, the electrolyte within the housing will absorb water from the air through the cathode and fail due to a condition called "flooding". Flooding may cause the battery housing to burst or leak. If the ambient humidity is less than the humidity within the battery housing, the metal-air battery will release water vapor from the electrolyte through the air cathode and may fail due to "drying out".
The art, therefore, has recognized that an ambient air humidity level differing from the humidity level within the battery housing will create a net transfer of water into or out of the battery housing as the respective humidity levels seek equilibrium. The transfer of humidity is particularly of concern when the battery is not in use. If uncontrolled, humidity will tend either to seep into or out of the battery housing over an extended period of time. This transfer of humidity will shorten the shelf-life of the battery as a whole or cause some or all of the cells to fail.
Maintaining a battery housing with consistent levels of humidity during non-use generally has required a sealed housing. Prior art systems, such as that discussed above by Cheiky, have used a fan of some sort to force ambient air through large openings in the battery housing during use. These openings are then sealed with a mechanical air door during non-use. If the air door is not present or is not shut during non-use, large amounts of ambient air will seep into the housing. This flow of air can cause the humidity transfer problems with the housing discussed above. The oxygen in the ambient air also may cause the cells to discharge, thereby leading to "leakage" current and a reduction in cell efficiency and lifetime.
The assignee of the present invention is also the owner of application Ser. No. 08/544,707, entitled "Diffusion Controlled Air Door," filed Oct. 18, 1995, now U.S. Pat. No. 5,691,074, and application Ser. No. 08/556,613, entitled "Diffusion Controlled Air Vent and Recirculation Air Manager for a Metal-Air Battery," filed Nov. 13, 1995, now U.S. Pat. No. 5,919,582 (attached hereto as Exhibit A). These references disclose several preferred metal-air battery packs for use with the present invention and are incorporated herein by reference.
These metal-air battery packs have a housing with an air inlet opening and an air outlet opening both sized with a length in the direction through the thickness of the housing being greater than a width in the direction perpendicular to the thickness of the housing. The openings are unobstructed and are sized to eliminate substantially the air flow into the air inlet opening and out of the air outlet opening when the fan is turned off. By "eliminate substantially", it is meant that the rate of diffusion of oxygen, humidity, or contaminates through the openings is not a concern during non-use of the battery. The respective diffusion rates are so slow that the humidity transfer or drain current is sufficiently small such that they have little appreciable impact on the efficiency or lifetime of the battery.
Although both the mechanical air door systems and the open air door systems may prevent the transfer of humidity through the housing during non-use of the battery, these systems do not have a significant impact on the regulation of the humidity level during use. One method to limit the transfer of humidity during use is shown in commonly-owned U.S. Pat. No. 5,356,729 to Pedicini. This reference describes the use of ventilation openings that are preferentially sized for the diffusion of oxygen into the housing upon the reduction of the oxygen concentration within the housing during use. By preferentially diffusing oxygen into the housing, the need of the cells for oxygen is met while maintaining a more stable water vapor and humidity equilibrium within the housing. Although this disclosure somewhat reduces the transfer of humidity during use, this system does not eliminate such transfer.
Other known system have attempted to resolve this problem through the use of a recirculating air manager. For example, commonly-owned U.S. Pat. No. 5,560,999 to Pedicini, et al. describes an air manager system that recirculates the reactant air through the battery housing and exchanges only a minimal amount of the recirculated air for the ambient air to maintain a sufficient oxygen concentration in the battery. The amount of oxygen admitted may be increased or decreased depending upon the load on the battery. The battery housing also may include a humidifier in the recirculating air pathway to humidify the recirculated reactant air as determined by a humidity monitor.
Although these disclosed devices and methods are helpful in limiting the exchange of humidity into or out of the battery housing, these designs require monitoring devices and complex air manager systems to maintain the proper humidity levels. These designs can be expensive to manufacture and difficult to use. Further, these known devices do not completely eliminate the transfer of humidity during both use and non-use of the battery. Changes in the ambient humidity level will still have an impact on the operation of the battery regardless of the known safeguards.
What is needed, therefore, is a humidity control system that can accommodate varying humidity levels via a relatively simple air manager design. This air manager system must be capable of regulating the humidity level within the battery housing both during use and non-use without the need for complex monitoring systems and humidifiers.